A variety of pumps are known for pumping molten metal. One type of centrifugal pump typically includes a submerged base including an impeller chamber that receives a rotatable impeller. The impeller may include vanes and/or passages for directing the molten metal during pumping. The base is submerged in a well that contains molten metal in a furnace. A motor driven shaft is fastened to the impeller and rotates it. Rotation of the impeller causes molten metal to be drawn into the base inlet, through the impeller chamber and to flow through an outlet. The pump may be a circulation or transfer pump.
Impeller based pumps do not positively displace molten metal. There is not a close correspondence to the volume that is inlet compared to the volume that is outlet. Centrifugal pumps are somewhat inefficient. It has been a long felt but unresolved problem in molten metal processing to produce a charge of molten metal of defined volume. To pump at high pressure and/or to pump small charges of metal cannot be accomplished with this type of impeller based centrifugal pump.
Centrifugal pumps also operate at high rpm and often in a dirty environment. Debris that enters the impeller chamber can cause catastrophic failure of the impeller and/or shaft. Such pumps typically have a short lifetime in the harsh molten metal environment.
The molten metal furnace or vessel containing molten metal is often located relatively close to a die casting machine at the same facility. One modern way that molten metal is transferred to die casting machines is through the use of robots; which move to receive and transfer the molten metal in defined volumes for die casting metal parts of particular, often complex shapes. However, robots present very high capital and maintenance costs and sophisticated personnel to operate and modify the movements of the robots. Robots can also be dangerous as they operate quickly and at high force, which can injure workers. Of course robot malfunctioning is also a dangerous risk when the robots work with molten metal.
The molten metal processing industry is also demanding higher and higher purity metal parts. Impurities can result from the presence of oxides and other particles. For example, a part with a pinhole defect used in a transmission of a motor vehicle can ruin the resultant parts or lead to premature failure. This can be reduced by adding fluxes to the molten metal by a degassing operation and by physically skimming to remove dross. These are labor intensive and dangerous operations due to the addition of flux chemicals and gases to the molten metal and proximity of workers to the furnace of molten metal.